Many communication applications use multiple access, i.e., the ability to coordinate the use of a communication channel or medium among multiple transmitters. Several multiple access methods and systems are known in the art. For example, time division multiple access (TDMA) systems allocate different time slots to different transmitters. Frequency division multiple access (FDMA) systems assign a different frequency channel to each transmitter. In code division multiple access (CDMA) systems, each transmitter modulates its data with a different pseudorandom code or sequence, enabling the receiver to differentiate between the transmitters. Combinations of TDMA, FDMA and/or CDMA are also known.
Another multiple access method is described in U.S. Patent Application Publication 2004/0209570 A1, whose disclosure is incorporated herein by reference. A channel encoder converts alphabetic symbols into a pre-selected geometrical representation of a hexagonal lattice known as a symbol constellation. A channel decoder converts the received symbol constellation into a replication of the alphabetic symbols. Each user has a specially defined symbol set that allows a linear-complexity decoding operation.
Some multiple access receivers use multiuser detection methods for demodulating the mutually interfering signals originating from different transmitters. Such detection methods are described, for example, by Verdú in “Multiuser Detection,” Cambridge University Press, August, 1998, chapter four, pages 154-213, which is incorporated herein by reference.
Another multiple access technique is referred to as cooperative transmit diversity and is described, for example, by Larsson and Vojcic in “Cooperative Transmit Diversity Based on Superposition Modulation,” IEEE Communication Letters, (9:9), September, 2005, pages 778-780, which is incorporated herein by reference. The paper describes a method in which a transmitter relays the information of another transmitter in addition to its own information, in order to improve its error performance.